1. Field of the Invention
This invention relates to an optical recording disk having a dye film as a recording layer, and more particularly, to an optical recording disk of the write-once type using a phthalocyanine dye.
2. Prior Art
Optical recording disks of the write-once, rewritable and other types have been of great interest as high capacity information carrying media. Some optical recording disks use a dye film as the recording layer. From a structural aspect, commonly used optical recording disks are classified into the air-sandwich structure having an air space on a dye base recording layer and the close contact type having a reflective layer in close contact with a dye base recording layer. The latter has the advantage that they can be reproduced in accordance with the compact disk (CD) standard. In this regard, reference is made to Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and Proceedings SPIE--The International Society for Optical Engineering, Vol. 1078, pages 80-87, "Optical Data Storage Topical Meeting", 17-19, January 1989, Los Angels.
In the close contact structure having a reflective layer in close contact with a recording layer, the substrate is formed with a spiral groove at a pitch of about 1.6 .mu.m for tracking. The region of the recording layer in the groove serves as a recording track.
Among the dyes used in the recording layer, cyanine dyes were predominant in the prior art. However, the cyanine dyes are generally less resistant to light and insufficiently reliable on recording. Attempts are thus made to use phthalocyanine dyes instead. The phthalocyanine dyes, however, have the problem of low recording sensitivity when used in the recording layer because they are generally thermally stable.
One solution is proposed by Japanese Patent Application Kokai (JP-A) No. 255339/1993 which discloses the use of a nitrosilicon phthalocyanine having --OP(.dbd.O)(R.sup.1)(R.sup.2) ligands coordinated above and below a center atom Si wherein R.sup.1 and R.sup.2 are alkyl, aryl, alkoxyl or aryloxy groups. In order to provide high recording sensitivity, a nitrosilicon phthalocyanine compound of a specific structure having a selected type of substituent must be used. Some compounds are not commercially available and must be synthesized through a process which is cumbersome to the disk manufacturers. Even if synthesis is possible, some compounds are difficult to synthesize or low in yield.
It is also contemplated to increase recording sensitivity by modifying the shape of a groove in the substrate. Actually attempting such shape modification, we found that the problem could not be solved when phthalocyanine dyes were used as the recording layer.
Therefore, it is desired to find a simple means for improving the recording sensitivity of an optical recording disk using a phthalocyanine dye in the recording layer.